Embodiments described herein relate generally to improved systems and methods for the desalination of seawater and, more particularly, to methods and apparatus for the treatment and disposal of concentrate (brine) generated from brackish desalination plants by use in existing seawater desalination plants.
The desire to make drinkable, potable water out of seawater has existed for a long time. Several approaches can be taken to remove the salt and other chemicals. Voutchkov (U.S. Pat. No. 6,946,081) discloses a desalination system. The entire disclosure of U.S. Pat. No. 6,946,081 is incorporated herein in its entirety. Water distillation is one way to approach the goal, but may not be commercially feasible. In this approach, water is heated to separate the solids from the liquid and therefore remove the salt solids. Another approach is electrodialysis in which the ions forming the salt are pulled by electric forces from the saline water through membranes and thereafter concentrated in separate compartments. This approach is also very expensive. A third approach to desalination is through reverse osmosis. This method uses pressure to force salty feed water against membranes which allows the relatively salt free water to pass through, but not much of the salts or other minerals.
Brine (concentrate) is produced which is a by-product of part of a desalination system. Disposal of brine from brackish desalination plants is usually one of the key limiting factors associated with the wider implementation of inland brackish water desalination. Currently, in many locations worldwide brine from inland brackish desalination plants is disposed of most often by either deep well injection into high-salinity aquifers or by conveyance using a regional interceptor pipeline to a wastewater treatment plant (WWTP) and is discharged to the ocean using the treatment plant's ocean outfall. The first disposal method (i.e., disposal of concentrate to deep saline aquifers) is often limited and very dependent on the availability of such aquifer in the vicinity of the brackish desalination plants. In many cases such aquifers are not readily available. The second approach (i.e., brackish concentrate disposal through the outfall of an existing WWTP) is also very costly and more importantly, it occupies outfall capacity and thereby indirectly limits the treatment capacity of the host WWTP. Both alternatives treat brine from inland brackish desalination plants as waste and involve significant expenditures for the disposal of this brine. Currently, desalination of brackish water (i.e. water of total dissolved solids (TDS) concentration below 15,000 mg/L) is used widely for production of potable water and high-purity water for industrial applications. The brackish water desalination plants yield two streams: low-salinity product water and high-salinity waste stream (brackish brine or brackish concentrate). The brackish concentrate has to be disposed of or reused in an environmentally safe manner.
At present, the method most widely used for brackish concentrate disposal is direct or indirect discharge to surface waters without treatment (rivers, estuaries, ocean, etc.). Currently, more than 85 percent of the brackish desalination plants in the continental United States discharge their brackish concentrate to surface waters (See, for example, Mickley and Associates (2001) “Membrane Concentrate Disposal: Practices and Regulation”, U.S. Department of the Interior, Bureau of Reclamation, Technical Services Center, Desalination and Water Purification Research and Development Program Report No. 69). A study completed by the American Water Works Research Foundation (See, for example, Mickley, M. C. (2000) “Major Ion Toxicity in Membrane Concentrate”, Denver, Colo., American Water Works Research Foundation.) indicates that discharge of brackish concentrate to surface waters frequently exhibits toxicity to marine aquatic species and, therefore, in most cases is no longer a viable method for concentrate disposal. At present, the brackish concentrate toxicity is a known and well documented problem. The United States Environmental Protection Agency and most State environmental protection agencies require monitoring and treatment of brackish brine to detoxify it prior to disposal. Existing methods for concentrate treatment and disposal such as: evaporation in man-made ponds; deep well injection in hypersaline aquifers; or treatment by mechanical evaporation and crystallization are very costly and/or are applicable for very small brackish water desalination plants due to site constraints.
Research completed on the toxicity of membrane treatment plant concentrates (brines) from various water sources indicates that treatment processes that produce toxic brine are: groundwater and brackish water reverse osmosis treatment plants, membrane softening plants, electrodialysis desalination plants, and electrodialysis reversal treatment plants. (See, for example, Mickley and Associates, 2000). This research also concludes that brackish concentrate toxicity is not caused by the membrane treatment process itself but results from the nature of the groundwater/brackish water source and its major ion makeup. In comparison to brackish concentrate, high-salinity concentrate generated during the desalination of seawater by reverse osmosis membranes does not exhibit toxicity (See, for example, Voutchkov, N. (2007) “Novel Method for Assessing Salinity Tolerance of Marine Organisms”, Environmental Engineer: Applied Research and Practice, Summer 2007, pp. 24 to 28, American Academy of Environmental Engineers).
Each of these efforts may increase the efficiency of the desalination system, but these efforts may not sufficiently reduce the cost of the system for use for public water supply. What is needed is a desalination system that processes seawater into potable water more cost effectively for use for public water supply, which includes the disposal of brackish concentrate.